CO2 in solar collectors??

[Solar Mike]

Originally Posted by AC_Hacker

Mike, do you have more info on this? The amount of heat isn't such a big surprise, but the distance (10 meters or about 30 feet) is big news to me.

-AC_Hacker

I have made several longish heat pipes, one was 6m long made from 6mm (1/4") copper tubing, heating one end, the other was hot within seconds.
Similarly a heat pipe made from 15mm copper tube 5m length behaved the same. The length of the pipe between hot and condensing ends has to be insulated.
The speed of heat transfer is determined by the vapor velocity of the gas, obviously the whole pipe gets hot when heating at one end, so provided the whole thing is well insulated no heat is lost. The amount of heat that can be moved is determined by the dia of the pipe, the vapor velocity and the latent heat of vaporization of the media.

The 10m length whilst I have not made one this long, should work as successfully at the shorter versions.

I was experimenting making a heat transfer device for plugging in to the waste heat going up the flue from my pellet wood burner. My intention was to place a preheat water tank up in the roof space directly above the hot water cylinder, when the fire was going, the heat pipe would capture some of the heat going up the chimney and conduct it to the water tank approx 10 meters away. This would be less complex than putting a wetback in the fire, the heated end of the heat pipe could get up to the same temp of the exiting gases from the fire, which is a good thing as the pipe wouldn't get carboned up the same way one filled with water.

I have tried several media inside the pipe, Butane, water, R22. Butane didn't work that well, we cannot get pure propane here so couldn't try that. Water worked but is very difficult to evacuate the tube to < 100 microns, then put some water in it, R22 worked best as standard refrigeration techniques can be used to evacuate and inject the liquid gas.

This DIY method of construction only conducts heat upwards, as upon condensing, the media in the pipe has to drain back to the heated end. Commercial devices can have a wick that by capillary action draws the condensed liquid to the heated end, so these would work at any angle.

Mike

[AC_Hacker]

Originally Posted by Solar Mike

I have tried several media inside the pipe, Butane, water, R22. Butane didn't work that well, we cannot get pure propane here so couldn't try that. Water worked but is very difficult to evacuate the tube to < 100 microns, then put some water in it, R22 worked best as standard refrigeration techniques can be used to evacuate and inject the liquid gas.

All very interesting.

So what technique are you using to create a vacuum in the tube prior to introducing your refrigerant?

I tried to make a heat tube using R134a, which is similar to R22. I didn't have much luck.

-AC_Hacker

[Solar Mike]

Originally Posted by AC_Hacker

All very interesting.

So what technique are you using to create a vacuum in the tube prior to introducing your refrigerant?

I tried to make a heat tube using R134a, which is similar to R22. I didn't have much luck.

-AC_Hacker

I have a standard vacuum pump goes down < 50 microns, standard multi way connection hub and hoses etc and a digital gauge so you can measure it with any accuracy.

As the amount of liquid gas in the tube is small, say 100cc in a 10m pipe you need to construct a small charge tank that can be filled from the main bottle of refrigerant. Once evacuated, the contents of the charge tank fills the heat pipe.

Ideally the gas should be a single component, not a mixture of gasses.

Cheers
Mike

[AC_Hacker]

Originally Posted by Solar Mike

I have a standard vacuum pump goes down < 50 microns, standard multi way connection hub and hoses etc and a digital gauge so you can measure it with any accuracy.

As the amount of liquid gas in the tube is small, say 100cc in a 10m pipe you need to construct a small charge tank that can be filled from the main bottle of refrigerant. Once evacuated, the contents of the charge tank fills the heat pipe.

Ideally the gas should be a single component, not a mixture of gasses.

Cheers
Mike

Mike, this is really great information.

Do you happen to have a photo or drawing of your charge tank setup?

-AC_Hacker

[AlanE]

Originally Posted by AC_Hacker

Mike, this is really great information.

Do you happen to have a photo or drawing of your charge tank setup?

I second that.

[Solar Mike]

Originally Posted by AC_Hacker

Mike, this is really great information.

Do you happen to have a photo or drawing of your charge tank setup?

-AC_Hacker

Its been over a year since last playing with this stuff, so sorry no photo's, will do my best to describe:

The last heat pipes I was making for another project, required approx 50cc of liquid R22. It was a 20mm dia pipe 1200mm long with a number of internal 6mm tubes to carry water, this heat pipe was designed for horizontal orientation giving true double barrier isolation between the water and the outer wall of the pipe. The condenser end in this instance was the internal tubes

I made a small tank from a short length of 25mm copper tube, brazed end cap on each end and a 600mm length x 6mm tube at one end with flared fitting to connect to the pot of R22. Other end I welded a male termination connector direct to the end cap.

With the standard 3 lead HVAC gauge setup with flexi hoses, connect the low pressure port to your vacuum pump, the middle to the heat pipes schrader valve termination, the high pressure port connects to the termination on the small tank, the other end of the tank connects to the outlet fitting on the pot of gas.

With initially all needle valves closed, open both low pressure and high pressure.
Turn on vacuum pump, put your digital gauge on the system to monitor vacuum, the standard gauges on the 3 port valve assembly are next to useless for this.
There will be moisture inside the pipes, hoses and heat pipe, once vacuum < 100 microns this will evaporate at room temperature.
After about 15 minutes close the valve to the vacuum pump and monitor with the digital gauge, if there is still moisture in the setup the pressure will start to rise, or if you have a leak, keep repeating this step until its all ok.
Remove or shut off the digital gauge, high pressure will kill it.
Close the valve on the high pressure port, open the valve on your pot of gas for a minute to fill the line and small tank.
Close the valve on the gas pot.
At this point the small tank will be very cold, you want to wait for a while to all it to come to room temp, or shortcut the process and quench with a sponge of warm water. The pressure will rise a bit but the lines are flexible so that's ok.
Open the high pressure port needle valve and fill the heat pipe, make sure the header tank can drain the liquid gas into the heat pipe ,this is the highest point in the heat pipe as you want all liquid gas to flow to the bottom away from the schrader fitting.
Quickly decouple the flexi hose off the heat pipe, a little gas will escape, not enough to worry about. If this fitting was at the other end then liquid gas would gush out over your fingers, it boils at -40c so its instant frost bite, be very careful.

Cap the valve on the heat pipe and test it.
Tip it up, with your ear to the tube you will hear the liquid gas running down the tube.
With you ear to it, grip the tube at the end where the liquid sits, you will hear it start to boil after a few seconds.
Make a cup of tea, place the cold end into it, the top of the tube will be hot instantly, wiggle it in the tea, after a few seconds it will be cold, all the heat is now in the copper pipe.

The smaller the heat pipe the more difficult to get the correct amount of gas into it and the more critical leaky fittings are.

Use R22 when you want to pipe to work from 0c - 150c , its critical temp is 205 so is no good for high temp operation
For conducting heat from solar panels, perhaps water or acetone would be better, I have used acetone, but it catches fire very easily and you cannot see it burning, dangerous.
The only reliable way I can see to put water into an evacuated tube is place the water inside something that when heated will melt, releasing the water. I have tried putting water in the tube then freezing one end whilst attempting to evacuate the tube, doesn't work the ice vaporizes and also traps tiny air bubbles.

Hope that helps.
Cheers
Mike